The invention is based on a pressure valve as defined hereinafter. In such a pressure valve, known from Swiss Pat. No. 394 710, the prevention of post-dripping or post-injection of fuel after the end of the effective supply stroke of the injection pump piston is to be attained by providing a return-feed collar on the pressure valve, in combination with a relief throttle, which may as needed be controlled by a pressure maintenance valve. Without this provision, pressure waves in the fuel supply line between the pressure valve and the injection valve, which travel back and forth between the injection valve and the pressure valve, would arise after the end of the effective supply stroke of the pump piston and the subsequent closure of the pressure valve and the high-pressure injection valve. The waves reflected from the pressure valve to the injection valve and back are capable of opening the injection valve after the proper time, causing fuel to escape and resulting in the known disadvantages of such a phenomenon.
In this known pressure valve, the use of a return-feed collar alone is effective in preventing this disadvantageous effect only in the case of small injection quantities per injection stroke of the pump piston. The known embodiment has the sole purpose of preventing post-injection or post-dripping, in fact over a wide range of injection quantity control per injection stroke. In this embodiment, at times when the relief throttle is not controlled by a pressure maintenance valve, has the effect that the residual pressure in the fuel supply line varies severely with the rpm. With small fuel injection lines, the supply line between the pressure valve and the injection valve is relieved in the same manner as with large injection quantities. During the ensuing supply stroke, there is then a displacement volume of variable quantity, which must be filled by the pump piston feeding action until the injection pressure at the injection valve is attained. Especially with small injection quantities, this has the disadvantage, first, of notable deviations in fuel quantity because of the variable residual pressures, which impairs the smoothness of engine operation, and second, that part of the useful stroke of the pump piston is lost, or becomes ineffective.
This becomes particularly disadvantageous if the fuel injection pump is equipped with a so-called quiet operation device, which is intended to reduce the pumping rate of the pump piston. To this end, in a known manner, a portion of the fuel pumped by the pump piston is withdrawn, and the duration of pumping is lengthened in order to attain the desired injection quantity. For a long pumping duration, on the other hand, a long pump piston supply stroke is necessary for this quiet operation range, which as a rule corresponds to the engine idling range. Now if a portion of the effective supply stroke of the pump piston is required to fill the displacement volume, then the required total stroke of the pump piston for adhering to even large full-load injection quantities must be designed to be quite long, which is disadvantageous in terms of the structure and size of the fuel injection pump.
It is also known, from U.S. Pat. No. 2,706,490, in a pressure valve which is provided with a return-feed collar by means of which the fuel supply line can be relieved after the end of injection, for a throttle connection to be provided on the return-feed collar. A pressure valve closing member embodied in this way is provided for internal combustion engines which are to be driven with liquid and gaseous fuel simultaneously. The liquid fuel is injected in a very small quantity, as igniting fuel, into the combustion chambers of the engine and made to ignite. As a result, the gaseous charge is then ignited as well. At the same time, however, such an engine is also intended to be drivable with liquid fuel over its entire operating range. The throttle connection on the pressure valve closing member is intended to prevent the valve closing member from lifting, during the injection of igniting fuel, by the amount that would correspond to the relief volume without the throttle. Thus the intent is to prevent the fuel supply line from being relieved, at the end of injection, by the full amount of the relief volume made available by means of the relief collar. This relief would affect operation in an unsatisfactory manner. By means of the throttle connection, the relief is to be precluded completely in the operating range in which igniting fuel is to be injected, in that the igniting fuel quantity bypasses the relief collar via the throttle connection without effecting a significant pressure drop. Contrarily, during operation with liquid fuel only, the relief is intended to be fully effective by means of the return-feed collar.
This arrangement has the disadvantage, however, that it does not allow variation of the injection quantity over a very wide variation range. With large injection quantities and operation entirely with liquid fuel, the effect of the return-feed collar is reduced considerably by the throttle connection. With large injection quantities per stroke and low rpm, high fuel supply line residual pressures can still arise, however, causing pressure waves which are reflected by the pressure valve and cause an uncontrolled opening of the injection valves. The fuel that belatedly escapes undergoes poor preparation, enters the combustion chamber too late, and no longer participates fully in the combustion process. This causes smoky combustion and carbonizing of the nozzles, and nozzle carbonizing in turn is associated with a further prolongation of the injection times. Accordingly, engine operation and combustion efficiency are impaired.